Heretofore, although polyvinyl chloride resin has been widely used as a flexible resin, development of its substitute is strongly desired as it is known that the polyvinyl chloride resin generates toxic compounds under combustion process.
Polypropylene based polymers are known as a substitute of the flexible polyvinyl chloride resin. Polypropylene based polymers are produced under various catalysts. However, the polypropylene based polymers produced by using the conventional catalysts have a drawback that a sticky component increases when it is made flexible (i.e., low modulus of elasticity). There is also possibility of having various problems such as deteriorating surface properties of the product when the polypropylene based polymer is molded, or when the formed sheets or films therefrom are used for food and medical applications. Moreover, there is a big problem in polypropylene based polymers that the glass transition temperature Tg is relatively high (about 0° C.) and the impact strength at low temperature (e.g., at −30° C.) is low.
On the other hand, 1-butene based polymer (polybutene-1) has been used in various applications as a general purpose resin because of its superior mechanical strength and a high heat resistance as well as its low price.
For example, as 1-butene based polymer has features such as a superior transparency, stiffness, heat resistance and a low hygroscopicity, it is used for biaxially stretched films or cast films such as laminate film. Furthermore, crystalline 1-butene based polymer film is widely used as packaging film utilizing its excellent rigidity, transparency and moisture proof property.
However, 1-butene based polymer film requires lower supercooling temperature for initiating crystallization and has lower crystallization temperature than polyethylene based polymer, even though the melting point is similar. Those tendencies are more remarkable in less crystalline polymers such as copolymer and the polymers with lower stereoregularity. Those characteristics lead to a difficulty of forming, inferior resin property, low temperature sealing property, modulus of elasticity and impact strength.
1-butene polymer has heretofore produced using titanium catalyst deposited on magnesium carrier (Japanese Patent Application Laid-Open 7-145205), but the composition of the polymer was not homogeneous leading to inferior properties such as stickiness and low transparency. Recently, by use of metallocene catalysts, 1-butene polymer with a homogeneous composition is obtained (Japanese Patent Applications Laid-Open Nos.62-119214, 62-121708, 62-119213, 8-225605). However, the homopolymers disclosed in those prior arts had a high stereoregularity and was lack of flexibility. In order to increase flexibility, a copolymer of 1-butene with an α-olefin was proposed. However, even though using the metallocene catalyst, in case of a simple copolymer of 1-butene, the composition sometimes was not quite homogeneous so that it was difficult to effectively prevent stickiness or inferior transparency.
Moreover, it is known that, in general, 1-butene based polymers show different crystal structures called Type I crystal and Type II crystal according to X-ray diffraction analysis. The conventional 1-butene based polymers had a disadvantage of having a shrinkage of product due to a gradual transformation of the crystal structure from Type II crystal to Type I crystal.
On the other hand, recently, olefin based polymer produced by using a metallocene catalyst is proposed as a substitute for flexible polyvinyl chloride resin. Linear low density polyethylene (LLDPE) produced using metallocene catallyst is an example. Although such a LLDPE has certainly a flexibility, it has low transparency and inferior surface property, therefore, it was not sufficient. Moreover, there is a problem of difficulty in improving the properties due to its poor compatibility with other α-olefin based polymers. Therefore, as a substitute for flexible polyvinyl chloride resin, development of a polyolefin based resin having an improved balance between the tensile modulus of elasticity (heretofore, referred simply as modulus of elasticity) and amount of sticky component, as well as an excellent impact strength, has been desired.